This is a carry over issue from KSP1. Aircraft can be very temperamental to fly, especially on the keyboard. KSP also tends to have very harsh induced drag if angle of attack gets too high. If you have an efficient low thrust plane, you may find yourself in a situation where you can't accelerate past a certain speed because the controls are deflected too much and make the plane too draggy. If it was easier to get partial deflection, this would be a problem less often. Another issue is over G. A really agile plane can easily rip itself apart if you pull to hard.
There is the deflection limit for control surfaces, but changing these mid flight, especially for multiple surfaces, is not practical for the issues I've outlined. I do not have a perfect solution for this problem, but I do have one suggestion that might be refined into a solution.
A Fly by Wire control part. When added to a plane it allows tuning of the controls vs speed/altitude/G load/other factors. This way control surface max deflection can be programmed by an aircraft designer to make flying more predictable. For example, limit control deflection to 20% below Mach 2 and raise it to 40% above Mach 2. Or limit aircraft G load to 4 G, controls will deflect to maximum until that G limit is hit. Simple interpolation between points may also be useful. For example 10% deflection at sea level and 100% at some specified altitude with a linear interpolation or curve between the two end points that continually varies the deflection limit.